Rapid industrialization of societies has led to a general increase in the production, application, and disposal of a broad spectrum of synthetic organic compounds. Most anthropogenic compounds find their way into surface and subsurface waters through industrial waste streams. Many anthropogenic compounds are hazardous, manifesting toxicity, carcinogenicity, and other insidious effects on human health. Development and application of cost-effective remedial technologies for protecting our limited water resources has therefore become an issue of great importance in recent years.
Different types of synthetic organic compounds have been encountered in groundwaters, among which volatile organic compounds such as trichloroethylene (TCE) and perchloroethylene (PCE) have been commonly identified in most surveys conducted across the United States. The U.S. Environmental Protection Agency (EPA) reviewed a number of strategies for aquifer remediation and treatment of waters polluted with volatile organic compounds, and recommended packed-tower air stripping and granular activated carbon adsorption as the best available technologies.
Packed-tower air stripping involves feeding a column with water from the top of the column and bubbling air from the bottom of the column through a packed bed. The air contacts the water and removes volatile organic compounds from the water by volatilization.
Air stripping alone is not efficient, especially for synthetic organic compounds. Moreover, the pollutants removed from the water by this process are released into the atmosphere and cause air pollution. Air stripping combined with granular activated carbon adsorption of the off-gases removed by air stripping is efficient but very expensive.
Granular activated carbon fixed-bed adsorption involves passing the water through granular activated carbon. It is expensive and not economically feasible for small systems. Both granular activated carbon fixed-bed adsorption and packed-tower air stripping require some pretreatment (such as coagulation, flocculation, sedimentation, and filtration) to remove suspended and colloidal matters, otherwise system fouling will increase operation and maintenance costs. Thus, there exists a need for an efficient and economical system for removing contaminants from water supplies.
One such contaminant in many water supplies which has captured much public concern is radon. Radon is a radioactive gas found mainly in ground water supplies. Radon concentrations as high as 750,000 pCi/L (picoCuries per liter) have been observed in public water supplies. Although there is currently no government standard for the maximum contaminant level for radon, the United States Environmental Protection Agency (EPA) has proposed adopting a maximum contaminant level for radon of 300 pCi/L.
Nearly 80% of the water for the Nation's 60,000 public water supplies come from groundwater sources. Thus, the risk of exposure to radon is immense. If the maximum contaminant level for radon proposed by the EPA is adopted, 30,000 drinking water utilities will be out of compliance. A greater risk of exposure to radon gas is imposed on populations served by smaller water utilities that obtain their water from small aquafiers which usually contain higher concentrations of radon.
Radon gas escapes from water at the point of use (i.e., at the faucet), thereby increasing indoor radon concentration levels to 10-20 times higher than that of outdoor concentrations of radon gas. Epidemeologic studies have shown that inhaled radon leads to lung cancer. However, recent studies have indicated that the number of fatal cancers attributed to radon ingestion from drinking water may be equal to the fatal lung cancers caused by radon inhalation. The lifetime risk due to exposure to radon is two orders of magnitude higher than that from natural uranium. Additionally, radon is responsible for 80% of the radionuclide-induced deaths in the United States.
Presently there is no acceptable method for removing radon from groundwater. Air stripping is unacceptable because it releases the removed radon into the atmosphere. Granular activated carbon absorption is similarly unacceptable because the carbon becomes contaminated with radioactive radon, thus creating a large disposal hazard. Additionally, granular activated carbon is expensive and not economically feasible for small systems.
U.S. Pat. No. 4,610,792 to Van Gils et al. discloses a technique integrating an ultrafiltration membrane process and powdered activated carbon adsorption for the removal of emulsified oil from laundry wastewaters. The adsorption technology disclosed, however, is inadequate for purification of waters contaminated with volatile organic compounds which will be used for human consumption.
Therefore, there exists a need for a non-polluting, economical system for effectively removing radon and other contaminants from water supplies.